1. Field of the Invention
The present invention relates to a mode transducer for converting modes of different orders but of the same polarization to enable coupling between a pair of optical wave guides.
2. Prior Art
In optical transmission systems, it would be desirable to combine multi-mode and monomode systems. It is possible to combine in the certral control a plurality of multi-mode systems each having small band widths to form a monomode or single mode system having a large band width and then to transmit the combined optical systems over long distances to another central control where the monomode system is split up into a plurality of narrow band multi-mode systems. It is also desirable to have available a multi-mode system which can be connected to integrated optical components. Since integrated optical components are considerably easier to produce in either a monomode design or for a few lower modes and since many components can be fundamentally designed only for monomode operation, transducers are required to convert a plurality of modes to one or more than one low mode and vice versa.
Both transducers, which convert a specific mode to another mode or which either pass or block one or more than one mode are desirable for filter purposes. A mode filter of this type can for example be used to separate the higher modes from the lower modes of a multi-mode fiber. The higher modes may be used to supply short subscriber lines and the lower modes which have both lower attenuation and transit time distorsions are transmitted over the longer distances.
The separation of the individual modes by means of mode filters, also, facilitates the compensation of transit time differences and attenuation differences of the individual modes in the light wave guides which have a low mode conversion so that a substantial large band width can be achieved.
Also, in many optical components such as parametric amplifiers and frequency doublers, generally anisotropic wave guides are coupled and each of these wave guides has a different mode being propagated therein. To obtain a phase matching between the different modes, a mode transducer is required.